Spectroscopic analyzers to analyze a sample by spectroscopy with ultraviolet, visible or infrared light, which are included in liquid analyzers such as a liquid chromatography analyzer, are provided with a flow cell, and these analyzers perform absorption spectrometry or fluorescence analysis using this flow cell. A flow cell with a longer optical path length typically shows higher analytical sensitivity. A flow cell with a long optical path length, however, has a problem that measurement light impinges on the inner wall of the channel, and so the amount of measurement light passing through the flow cell decreases and so the sensitivity deteriorates. Especially in the case of an analyzer based on absorption spectroscopy, measurement light impinging on the inner wall of the flow cell will cause disordered light reflection/light scattering or light absorption, thus increasing noise often (Non-Patent Literature 1). That is, there is a known problem that solution changes in refractive index because of external factors such as temperature change, pressure change or composition change of the solution, which changes the ratio of such reflected light or scattered light with respect to the incident light, and such a change is detected as apparent change in absorbance, i.e., noise or drift.
In order to solve these problems, a method to totally reflect measurement light at a wall face of the channel has been proposed for improved sensitivity. This method can be roughly classified into two types of including a light reflective layer at an inner wall of the channel and including a light reflective layer at an outer wall of the channel. For the former method, a method proposed is to apply an organic material having a refractive index lower than that of water (refractive index 1.33) that is the most frequently used for spectroscopic analysis, particularly Teflon (registered trademark) AF as a fluorine polymer, to the inner wall of the channel as a light reflective layer (Patent Literature 1, Patent Literature 2). Such a structure achieves total reflection of measurement light 104 at the interface between solution 103 in the flow cell and a light reflective layer 102 disposed at a channel inner wall 101 as shown in FIG. 1. This method unfortunately fails to keep the total reflection conditions of the light if the inner wall of the channel adsorbs dirt, and so sensitivity deteriorates. Additionally, it is difficult to apply an organic material of a low refractive index that is required for total light reflection at the inner wall of the channel in a flow cell having a long optical path length so as to exert such optical characteristics, and so there is upper limit for the length of an optical path that can be increased.
For the latter method, a proposed method is to use a flow cell having a channel that is made of a glass capillary that does not absorb measurement light and dispose a light reflective layer at the outer face of the glass capillary, whereby as shown in FIG. 2, measurement light 104 propagating through solution 103 is totally reflected at the interface between the outer face of the glass capillary 201 and the light reflective layer 202. Since this is total light reflection based on a difference in refractive index between the solution and the light reflective layer via the glass capillary, and as its advantages, it is easy to lengthen the optical path, and there is no upper limit in theory for the length of the optical path that can be increased. For instance, a method proposed is to use air as the light reflective layer to realize total reflection of measurement light at the interface between the outer wall of the channel and air (Non-Patent Literature 2). FIG. 3 shows a typical structure of such a flow cell. The flow cell as a whole is configured to include the glass capillary 201, two optical fibers 302, 303, on which measurement light is incident for receiving, two pipes 304, 305 to introduce and discharge solution, and joint parts 306 and 307 having two channels to joint them. Measurement light propagates through the flow cell while being totally reflected at the interface between the outer face of the glass capillary 201 and air due to a difference in refractive index between solution and air. As another structure proposed is a flow cell including a light reflective layer provided at the surface of a glass capillary made of Teflon (registered trademark) AF (Patent Literature 3). When manufacturing these flow cells, the joint parts have to be connected without leakage of liquid, the joint parts including a channel to connect the flow cell with a glass capillary, an optical fiber and a pipe. To this end, there are known methods to apply pressure to them using mechanical components such as a ferrule and a nut to block a gap between the components and to provide a joint part made of a thermal-dissolving resin and apply heat to it, thus blocking a gap between the components with the dissolving resin for connection. As compared with the former, the latter method reduces the number of components and man-hours, and so the latter thermal welding is preferably used.